Ignition start of conventional discharge lamps generally adopts two approaches, i.e. hot cathode preheating start and cold cathode instant start. The discharge lamps adopted the cold cathode instant start include various types of high intensity discharge lamps and a cold cathode fluorescent lamp (CCFL) used as a backlight source for LCDs. Their start process is performed as follows: an ignition power source supplies electric power, and a ballast applies a high voltage to the electrodes at two ends of the lamp tube to ionize gas filled in the lamp tube to complete lamp ignition. Hence a rather high voltage must be provided to generate free electrons at a sufficient amount to conduct discharge current between the two electrodes.
Refer to FIG. 1 for a conventional hot cathode preheating start discharge lamp (commonly used for general fluorescent lamps or germicidal lamps). It includes a lamp tube 1 and two discharge electrodes 2 at two sides of the lamp tube 1. The lamp tube 1 is filled with a substance inside for discharging to emit light. Each of the two discharge electrodes 2 is connected to a tungsten filament 3 which is coated with a material having a low work function to easily emit electrons, and the material is commonly called electronic powder or cathode material. When ignition is started, the tungsten filament is electrically energized and heated to a temperature to emit a great amount of thermal electrons, then the ballast sends a high voltage between the two discharge electrodes 2 to generate discharge conduction to ignite the lamp tube 1 to complete ignition start process.
The lifespan of the hot cathode preheating start discharge lamp mainly depends on the two discharge electrodes. In the event that the electronic powder on the tungsten filament is consumed and exhausted by a great deal of sputtering or evaporation during the start process, or the tungsten filament is fractured to disconnect the heating power source, the discharge lamp cannot be ignited and becomes dysfunctional. In addition, each ignition process causes fierce impact of the electrodes by positive ions, which results in great sputtering of the electronic powder and huge consumption thereof. Empirical data show that each ignition process reduces the lifespan of the lamp by about 2-3 hours. On the other hand, a cold cathode discharge lamp has a non-helical tungsten filament for the electrode structure. Its ignition does not require a pre-heated process on the electrodes, but by directly applying a very high voltage on the two electrodes to generate very high electric field intensity between them to produce instant high voltage discharge to ignite the lamp tube. Hence a higher starting voltage is needed for lamp ignition. This creates a safety concern when in use. Compared with the cold cathode instant start discharge lamp, the hot cathode lamp has advantages of a lower starting voltage, lower energy consumption and greater safety. How to overcome the problems of the lower lifespan of the hot cathode discharge lamp and higher ignition starting voltage of the cold cathode discharge lamp is still an issue remained to be solved in the industry.